linux-kernel - Re: [PATCH] perf cpumap: Use scnprintf instead of snprintf

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Date:   Mon, 23 Mar 2020 14:03:34 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     Christophe JAILLET <christophe.jaillet@...adoo.fr>
Cc:     peterz@...radead.org, mingo@...hat.com, acme@...nel.org,
        mark.rutland@....com, alexander.shishkin@...ux.intel.com,
        jolsa@...hat.com, namhyung@...nel.org, kan.liang@...ux.intel.com,
        zhe.he@...driver.com, dzickus@...hat.com, jstancek@...hat.com,
        linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org
Subject: Re: [PATCH] perf cpumap: Use scnprintf instead of snprintf

On Sun, Mar 22, 2020 at 06:25:23PM +0100, Christophe JAILLET wrote:
> 'scnprintf' returns the number of characters written in the output buffer
> excluding the trailing '\0', instead of the number of characters which
> would be generated for the given input.
> 
> Both function return a number of characters, excluding the trailing '\0'.
> So comparaison to check if it overflows, should be done against max_size-1.
> Comparaison against max_size can never match.
> 
> Fixes: 7780c25bae59f ("perf tools: Allow ability to map cpus to nodes easily")
> Fixes: a24020e6b7cf6 ("perf tools: Change cpu_map__fprintf output")
> Fixes: 92a7e1278005b ("perf cpumap: Add cpu__max_present_cpu()")
> Signed-off-by: Christophe JAILLET <christophe.jaillet@...adoo.fr>
> ---
>  tools/perf/util/cpumap.c | 39 ++++++++++++++++++++-------------------
>  1 file changed, 20 insertions(+), 19 deletions(-)
> 
> diff --git a/tools/perf/util/cpumap.c b/tools/perf/util/cpumap.c
> index 983b7388f22b..b87e7ef4d130 100644
> --- a/tools/perf/util/cpumap.c
> +++ b/tools/perf/util/cpumap.c
> @@ -316,8 +316,8 @@ static void set_max_cpu_num(void)
>  		goto out;
>  
>  	/* get the highest possible cpu number for a sparse allocation */
> -	ret = snprintf(path, PATH_MAX, "%s/devices/system/cpu/possible", mnt);
> -	if (ret == PATH_MAX) {
> +	ret = scnprintf(path, PATH_MAX, "%s/devices/system/cpu/possible", mnt);
> +	if (ret == PATH_MAX-1) {

This should be a static analysis warning.

But isn't this stuff userspace?  I can't figure out how to compile it on
Debian so I'm not sure.  There is no scnprintf() in user space.

regards,
dan carpenter